Create 2 variables in the object.
Name one "colour" and set each instances value in the editor to "red" or "blue".
Name the other variable "overlapping_instance_ID".
add the event
object ball overlapping ball
set "overlapping_instance_ID" to unique ball ID.
Create a subevent, and set the condition to
pick instance by unique id, and set the value to the variable "overlapping_instance_ID".
Now add another condition to the event, ball variable "colour" = "red", set action to destroy ball.
Alternatively you can use another object as a detector, do a for each loop on all balls named blue and set the detector to their position, then do another event and if detector overlapping ball and ball colour is red then destroy ball.